Measuring Aromaticity with the Dimethyldihydropyrene Ring Current Probe. Experimental and Computational Studies of the Fulvenes and the Strongly Antiaromatic Cyclopentadienone Reveal Large Mills−

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• 作者：Reginald H. Mitchell ; Rui Zhang ; David J. Berg ; Brendan Twamley ; Richard Vaughan Williams
• 刊名：Journal of the American Chemical Society
• 出版年：2009
• 出版时间：January 14, 2009
• 年：2009
• 卷：131
• 期：1
• 页码：189-199
• 全文大小：398K
• 年卷期：v.131,no.1(January 14, 2009)
• ISSN：1520-5126

文摘

The synthesis of the methylfulvene- and phenylfulvene-annelated dihydropyrenes 10 and 22 from the cyclopentadiene-fused dihydropyrene 7 in 68% and 80% yields, respectively, are reported. However, the attempted formation of the parent fulvene-fused dihydropyrene 18 failed, both from the cyclopentadiene 7 with formaldehyde and from the cyclopentadienone 5 in Wittig-type reactions. Chemical shift data for the methylfulvene (35) and phenylfulvene (36)-fused dihydropyrenes 10 and 22 were used to estimate the reduction in the dihydropyrene nucleus aromaticity (DHPN) (relative to benzene fusion) in 10 and 22 (12−16% and 22−25% respectively). Calculations revealed that this reduction in diatropicity, contrary to the situation with benzene fusion, is not due to any aromaticity of the annelating fulvenes but instead is caused by Mills−Nixon-type effects. We conclude that methyl- and phenylfulvene are nonaromatic. An improved synthetic route to the cyclopentadienone 5 was found in an unprecedented cyclization of the trans-cinnamic acid analogue 29 in 80% yield. This enabled an X-ray structure of 5 to be obtained, for comparison to that of the saturated ketone 4. Even though crystals of 5 and 4 show diastereomeric disorder, when the average bond length data of cyclopentadienone 5 is compared with those of cyclopentenone 4 and the parent and benzo dihydropyrenes 6 and 33, it is clearly evident that 5 has the opposite bond-alternation pattern, consistent with a [4n] fused annulene. From the bond length data, cyclopentadienone has ∼87% of the effect of a benzene ring on bond alternation, which is in reasonable agreement with the previously found NMR value (78%). Structure and nucleus-independent chemical shift calculations support these results.